In a known centrifugal pump the suction side is provided with a gas separator which may be a separate or integral part of the pump housing. The gas separator comprises a cyclone chamber with an inlet channel for the pump medium to be purified of gas or air, and a liquid outlet channel connected to the inlet of the pump, as well as a bleed housing situated at the upper end of the cyclone chamber and comprising an air duct removing the gas or air separated in the cyclone chamber. Such a pump does not, however, operate completely satisfactory because liquid sometimes blocks the air duct of the bleed housing, especially when more water enters the cyclone chamber than can be contained therein. Such a liquid blockage is completely detrimental to the operation of the gas separator, and a consequence of the presence thereof is that the water entering the inlet of the centrifugal pump contains undesired bubbles of gas or air, said bubbles interfering with the rotation of the impeller of the pump.